This invention relates to the field of cleaning the surfaces within vessels that have restricted points of entry, and in particular, the surfaces within oxygen converters and oxygen cylinders. These oxygen converters and oxygen cylinders are components of the onboard oxygen supply systems of aircraft. These oxygen cylinders may be high pressure or low pressure, and may be fixed or portable. The interior surfaces may be metal, including stainless steel. The restricted points of entry may prevent these surfaces from being cleaned by application of mechanical force or sonic energy. The contaminants to be cleaned from the surfaces include organic matter and particulates.
The oxygen supply systems on aircraft may comprise oxygen converters, cylinders, lines, regulators, molecular sieve oxygen generators (MSOG units), and other apparatus. The cleaning of these oxygen supply systems is required primarily to remove two types of contamination. The first type of contamination arises from organic compounds. These organic compounds include jet fuel, compounds that result from the incomplete combustion of jet fuel, hydraulic oil and special types of greases that are used in these oxygen systems. The second type of contamination arises from particles of dust and dirt, as well as particles of Teflon that are found in the greases that may be used in these oxygen systems, and from Teflon tape which may be used in the threaded connections of these oxygen systems. The particulates may be in a size range of about one to 300 microns, and more commonly, in a size range of about 2 to about 150 microns.
One component of an aircraft oxygen supply system may be an oxygen converter. An oxygen converter may be a stainless steel sphere within a second stainless steel sphere. There is a vacuum seal between the inner sphere and the outer sphere. Oxygen converters are reservoirs that convert liquid oxygen to gaseous oxygen that may be breathed by the crew and passengers of the aircraft. At the present time, oxygen converters are typically constructed in volumes of 75 liters, 25 liters, 10 liters and 5 liters. The inner sphere of an oxygen converter typically has one small opening at the top, and a second small opening at the bottom. Each opening may be about 0.25 inch in diameter. Each line between the inner sphere and the outer sphere may not be straight, which may further restrict entry into the inner sphere. The opening at the top is used to vent the converter of gaseous oxygen. The opening at the bottom is used to repetitively input liquid oxygen into the converter and subsequently to output liquid oxygen from the converter. As the liquid oxygen exits the oxygen converter through the small opening at the bottom, it travels through a coil, and a pressure drop occurs that turns the liquid oxygen into gaseous oxygen. A harness is used to connect the oxygen converter to the oxygen lines in the aircraft.
The prior art cleaning of oxygen converters usually involved the removal of the oxygen converter from the aircraft. The oxygen converter was cut open, cleaned, and welded back together. Each cleaning resulted in a decrease in the size of the oxygen converter. This cleaning could be carried out only about two times because of the precise size requirements for oxygen converters. In some aircraft, recent experience is that a new oxygen converter may be in use for an average of seven years before the first cleaning. However, only three or four years pass before the second and final cleaning. The average service life of an oxygen converter may be less than fifteen years.
Prior attempts have been made to clean oxygen converters without cutting open the oxygen converter. Some attempts have involved the use of chlorofluorocarbons, and have generally had unsatisfactory results. Aqueous solvents are unacceptable because they are difficult to remove from converters, and residual water may freeze and create a dangerous buildup of pressure which may destroy the converter. Water may destroy the probe assembly within the converter.
There are certain requirements for methods, compositions and apparatus for cleaning the surfaces within aircraft oxygen supply systems to remove such contaminants. The methods should be able to be carried out in a relatively short period of time. Preferably, the cleaning should be carried out with the removal of a minimum amount of the components of the oxygen system from the aircraft. The cleaning compositions should be non-aqueous, non-flammable, non-toxic, and environmentally friendly. The solvent of the cleaning compositions should be able to be used as a verification fluid that is circulated through the cleaned components in order to verify cleaning. The cleaning should achieve at least a level B of ASTM standard G93-96, which may be stated as less than 3 mg/ft2 (11 mg/m2), or less than about 3 mg. of contaminants per square foot of interior surface of the components, or less than about 11 mg. of contaminants per square meter of interior surface of the components. The method of ASTM standard G93-96 may not accurately determine the level of cleanliness in vessels with restricted entry.
The present invention comprises methods, compositions and apparatus for cleaning surfaces, and particularly, cleaning the interior surfaces of oxygen converters and oxygen cylinders. These methods, compositions and apparatus have certain features in common, and other features that may be varied depending on the nature of the surfaces to be cleaned.
The present invention achieves the satisfactory cleaning of contaminants from oxygen converters without the need to cut the oxygen converter open, by using controlled flash boiling of the cleaning composition within the oxygen converter. The cleaning composition is both released into the oxygen converter, and maintained in the oxygen converter, at a temperature and pressure sufficient to maintain boiling. The pressure may be below ambient and the temperature above ambient, depending on the cleaning composition. The boiling provides agitation that achieves satisfactory cleaning. Adequate agitation cannot be provided by sonic energy or mechanical means due to the configuration of the oxygen converter.
The cleaning composition comprises a fluorocarbon solvent. In a preferred embodiment, the cleaning composition further comprises a fluorosurfactant. The boiling point of the fluorocarbon solvent should be sufficiently higher than the boiling point of the fluorosurfactant, to allow the removal of the fluorocarbon solvent from the mixture after the completion of the cleaning.
The apparatus for cleaning oxygen converters comprises a surfactant tank to store surfactant, and to provide surfactant to a surfactant proportioner. The surfactant proportioner stores a fixed amount of surfactant until it is flushed by solvent into a solution tank. A solvent tank is provided to store solvent, and to provide solvent to a solvent proportioner. The solvent proportioner stores a fixed volume of solvent, and delivers the fixed volume of solvent to the surfactant proportioner. The resulting mixture of solvent and surfactant is delivered to the solution tank. The solution tank delivers a fixed volume of solution to a pressure tank. The pressure tank is provided with heaters to increase the temperature and pressure of the solution. A vacuum pump creates a vacuum within a vacuum tank. The cleaning apparatus is attached to the oxygen converter which is to be cleaned. A valve between the oxygen converter and the vacuum tank is opened, and the gas within the oxygen converter is evacuated. The first valve is closed, and a second valve is opened between the pressure tank and the oxygen converter. The pressure differential between the evacuated oxygen converter and the pressure tank causes the heated, pressurized solution to flow from the pressure tank into the oxygen converter, and boil within the oxygen converter. After the oxygen converter is filled to the desired level with cleaning solution, the second valve is closed and the first valve between the oxygen converter and the vacuum tank is opened. The cleaning solution boils within the oxygen converter. After completion of a sufficient time period of boiling, the first valve is closed between the vacuum tank and the oxygen converter. The solution from the oxygen converter is then diverted to a distillation unit. In a preferred embodiment, dry air is introduced into the top of the oxygen converter while the solution exits from the bottom of the converter. The distillation unit distills solvent, which is returned to the solvent tank. The remaining surfactant and contaminants in the distillation unit are removed and disposed of. If required for sufficient cleaning, a single oxygen converter may be subjected to repetitions of the controlled flash boiling. After completion of the controlled flash boiling, the oxygen converter is rinsed with solvent, and then purged with dry air to remove the solvent.
The same methods, cleaning compositions and apparatus may be used to clean oxygen cylinders.